Grafts et al. — 158— Water in Plants 



radius of 0.1 mm. with a pressure of about 0.015 atmosphere. This is equivalent to a 

 rise of about 15 centimeters in an open tube. More important than the actual forcing 

 of air into solution is the movement of liquid resulting from the shifting balance of 

 capillary forces between tubes of different sizes. When a stem is cut vapor may be 

 formed in a number of elements of varying diameters. If the gas is pure water vapor it 

 will condense completely and dye will enter and move along all conducting elements. 

 If air gets in through the cut or comes out of solution from the xylem sap, competition 

 between capillaries will cause readjustment after the first influx of dye and bubbles 

 may be seen to be moving in opposite directions in different elements as liquid from the 

 larger ones flows into the smaller to satisfy capillary gradients. 



5) A slight elastic expansion of the liquid itself occurs with the release of tension. 

 This is of a minor magnitude. 



Preston (1938) discussed the above possibilities and concluded that the 

 last two are relatively unimportant. He developed equations expressing the 

 rate at which dye would move into the xylem under the first three conditions 

 basing these upon Poisouille's law. He assumed that resistance due to 

 irregularities of the walls would not be appreciable. This contrasts with 

 the view of Dixon and Ewart, who considered that resistance in vessels 

 would reduce flow to one-half the expected amount. Poisouille's law is 

 based on streamline flow. When one considers the contour of vessel walls 

 and visualizes the turbulence which the pits and thickenings might cause, 

 he is forced to the conclusion that Dixon and Ewart were probably nearer 

 the truth than Preston. 



By fitting his experimental data to his formula, Preston concluded that 

 many vessels in the ash tree used were filled with gas under pressures rang- 

 ing between 0.4 and 0.9 atmospheres. Other vessels may have become in- 

 jected due to elastic expansion of vessels containing water under tension. 

 Gas-filled vessels are located deeper in the trunk than water-filled ones. 



Preston's conclusions were probably correct but they do not prove that 

 the vessels contained gas in the intact trunk. The difficulty with this type 

 of experiment, as may be easily confirmed under the dissecting microscope, 

 is that when the columns are under high tension they may be disrupted by 

 deformation of the walls before the vessels are actually severed. Hence, 

 water vapor may occupy many vessels by the time the opening is made and 

 the dye solution flows in. If the gas in the elements is pure water vapor, its 

 pressure at 20° C. would be about 17.5 mm. of mercury or approximately 

 0.023 of an atmosphere. The pressure difference would be practically one 

 atmosphere (actually 0.977) and inward flow should be rapid. The differ- 

 ence between this value and those quoted from Preston probably repre- 

 sents the role played by resistance due to turbulent flow along the tubes. 



When tensions in the xylem are extremely high it seems doubtful if 

 injection can ever be accomplished without some vapor bubble formation 

 as the very inertia of the dye solution would resist the extremely rapid 

 motion required to maintain liquid continuity as the elastic expansion 

 throughout the length of a stem caused recession of the sap from the cut. To 

 be convinced of this one has only to watch the formation of vapor in the 

 elements of a woody stem of a plant in a state of permanent wilting as the 

 bark is removed under a dissecting binocular. Although the stem will under- 

 go considerable bending and twisting, as soon as the tensile columns are 

 subjected to deformation by jarring, squeezing, or cutting the tubes, they 

 break and vapor occupies the vessels as the water columns recede from the 

 point of rupture. Many, often most, of the elements have vapor in them be- 

 fore the dye can flow in and make contact with the sap columns. 



Baker and James (1933) report that in the majority of cases investi- 



